1. Field of the Invention
The present invention relates generally to the fabrication of thin films and more particularly to a new and improved method of producing thin films of electro-optically active transition metal oxides by plasma deposition.
2. Description of the Prior Art
The phenomenon of electrochromism in tungsten trioxide (WO.sub.3) was first reported by S. K. Deb in Applied Optics, Supplement 3, Electrophotography, (1969) page 192. Electrochromism is known by those skilled in the art to be a color change in a material or material system caused by an applied electric potential. The color change is either the result of the formation of color centers or is caused by an electrochemical reaction that produces a colored compound. The intensity of coloration depends on the magnitude of the electric potential and the amount of time during which the material is exposed. Coloration by an applied electric potential may be either cathodic or anodic depending upon whether coloration commences at the cathode or anode, respectively. Although the transition metal oxides, when viewed as a class of materials, are generally known to exhibit electrochromism, these materials may or may not be electro-optically active depending on their method of synthesis. An electro-optically active material is one that optically responds to an applied electric potential when incorporated into an electrochromic device.
Electrochromic display devices in which amorphous WO.sub.3 thin films are utilized have been studied by Colton, et. al., "Photochromism and Electrochromism in Amorphous Transition Metal Oxide Films" Accounts of Chemical Research, Volume 11 (1978), pages 170-6; "Faughnan, et al"., "Electrochromic Displays Based On WO.sub.3 ", Display Devices, J. Pankove Ed. (1980), page 181, Springer Verlag, New York; and Beni, et al., Ion Insertion Electrochromic Displays Advances in Image Pickup and Display, volume 5 (1982) pages 83-136, Academic Press, New York.
Research relating to the application of the electrochromic effect in transistion metal oxides is being actively pursued because of its potential use in solar technology. DiPaola, et al. ["Electrochromism in Anodically Formed Tungsten Oxide Films," Journal of Electrochemical Society, volume 125, number 8, (1978) pages 1344-47] and Chemseddine, et al. ["Electrochromism of Colloidal Tungsten Oxide, Solid State Ionics, volumes 9 & 10 (1983) pages 357-362] disclosed techniques for preparing or synthesizing electro-optically active thin films of electrochromic tungsten oxide called anodization and sol-gel technology, respectively. Anodization is a process in which an electrochromic coating is anodically formed. Sol-gel technology produces an oxide coating by depositing a colloidal solution onto a substrate. The techniques just described are wet-chemical techniques which do not lend themselves to mass production because of their costs, their prospect of chemical pollution, and their relative incompatibility for fabricating solid-state electrochromic devices. Consequently, they have recently received only slight attention and investigation in the art.
In November 1984 C. M. Lampert ["Electrochromic Materials and Devices for Energy-Efficient Windows," Solar Energy Materials, 11 (1984), pages 1-27]reviewed two physical vapor deposition techniques for producing electrochromic thin films, namely vacuum evaporation and sputtering. These two techniques are presently preferred because they are relatively inexpensive in comparison to wet-chemical techniques and lend themselves readily to fabrication of solid-state electrochromic devices.
In the vacuum evaporation and sputtering deposition techniques, thin films of WO.sub.3 are deposited in a vacuum environment from sources of W or WO.sub.3 in an oxidizing atmosphere. In the vacuum evaporation technique, the source material is heated to a vapor pressure sufficient to cause evaporation and condensation of the material onto a substrate. In the sputtering technique, the source material is converted to the vapor phase by positive ion bombardment. Thin film deposits of the WO.sub.3 are formed in both cases by vapor condensation onto a substrate in the vacuum chamber. The term vapor pressure used here is the pressure exerted when a solid or liquid is in equilibrium with its own vapor.
Electrochromic devices with thin film WO.sub.3 deposits may or may not be electro-optically active depending on their deposition process and their deposition parameters. H. R. Zeller and H. U. Beyler ["Electrochromism and Local Order in Amorphous WO.sub.3," Applied Physics, 13, (1977), pages 231-237] found little or no electrochromism in their H.sup.+ and Li.sup.+ insertion-type devices prepared with reactively sputtered WO.sub.3 films from a W target in a partial pressure of Ar and O.sub.2 gas. Evaporated films prepared at substrate temperatures of 300.degree. C. also exhibited no electrochromism. Furthermore, electrochromic devices utilizing WO.sub.3 films prepared by spraying meta tungstic acid at substrate temperatures of 320.degree. C. were electro-optically inactive.
Both evaporation and sputtering techniques are intrinsically high-vacuum techniques requiring vapor pressures in the range of 10.sup.-6 to 10.sup.-3 torr. A torr is a unit of pressure that equals 1.316.times.10.sup.-6 atmosphere. Those skilled in the art recognize that sophisticated, expensive vacuums and other equipment are required to produce pressures in this 10.sup.-6 to 10.sup.-3 torr range. Moreover, those skilled in the art understand that these techniques require additional power sources for multiple targets and for substrate heating, which to foster deposition.
Plasma chemistry is a technique by which thin films can, be produced by synthesizing reaction products from several ionized gaseous reactants. Plasma as used in this invention, is an electrically neutral, highly ionized gas composed of ions, electrons, and neutral particles. Generally, plasma deposition occurs when an electrical discharge in a low-pressure mixture of volatile reactants causes the formation of a variety of highly energetic species, e.g., atoms, metastables, radicals, ions, and the like, which chemically interact to produce stable deposits.